The present invention relates to an electronic device substrate structure and particularly to an electronic device substrate structure which is provided with a thin film having a wurtzite crystal structure and formed on an Si single crystal substrate and which is applied to piezoelectric effect-using devices such as thin-film bulk vibrators, SAW (Surface Acoustic Wave) devices, etc.; semiconductor light-emitting devices such as LEDs (Light-Emitting Diodes), laser diodes, etc.; IC heat sinks; and optical devices such as optical modulators, optical switches, OEICs (Opto-Electronic Integrated Circuits), etc.; and an electronic device using the substrate structure.
An integrated electronic device having functional films of oxide, nitride, etc. formed on an Si substrate as a semiconductor crystal substrate has been invented. For example, an LSI higher in degree of integration or a dielectric isolation LSI by an SOI technique by combination of a semiconductor substrate and a dielectric thin film has been examined. When a thin film of a ferroelectric substance as a kind of dielectric is further used, it is possible to form a non-volatile memory, an infrared sensor, an optical modulator, an optical switch, an OEIC, etc. When a semiconductor substrate and a superconductor thin film are combined, it is possible to form an SQUID, a Josephson device, a superconducting transistor, an electromagnetic wave sensor, a superconducting wiring LSI, etc. When a semiconductor substrate and a piezoelectric thin film are combined, it is possible to form an SAW device, a convolver, a collimator, a memory device, an image scanner, a thin-film bulk vibrator, a filter, etc.
To achieve optimal device characteristic in the electronic devices using these functional films and keep the reproducibility thereof high, it is preferable that the crystallinity of the functional films is made as high as possible. ZnO or AlN widely known as a typical piezoelectric substance exhibits high piezoelectricity in a <0001> direction of crystal, that is, in a direction of a c axis because ZnO or AlN has a wurtzite crystal structure. In the electronic devices using these materials as functional films, it is therefore preferable that the functional films are provided as simple c face-oriented highly crystalline films.
Under such circumstances, the present inventors of the invention have made a proposal in JP-A-11-260835. In the proposal, a buffer layer of ZrO2 or the like is provided as a single crystal film (epitaxial film) on an Si single crystal substrate, a metal thin film of Pt or the like is further provided as a single crystal film on the buffer layer, and a wurtzite type thin film of ZnO, AlN or the like having a wurtzite crystal structure is further formed on the metal thin film so that the wurtzite type thin film is provided as a simple c face-oriented single crystal film.
In a thin-film bulk vibrator using an Si single crystal substrate, the Si single crystal substrate is anisotropically etched with a KOH solution or the like in a production process to thereby form a via-hole. For this reason, it is necessary to use an Si single crystal substrate having an Si(100) plane oriented in parallel to the substrate surface.